a) Field of the Invention
The present invention relates to a golf club, and more particularly to a golf club with an improved head.
b) Related Art
(1) A recent trend of the golf club is that the club head is made of a metallic material, and formed with a shell having a hollow interior. Increase of the club head size and thinning of the face portion of the club head and other tendency progress. The theory teaches that the size increase of the club head accrues to the increase of a moment of inertia and the enlargement of the sweet spot, and hence to the stabilization of the flying direction of a ball, and that the thinning of the club face accrues to reduction of the weight of the whole golf club, and hence to the increase of a flying distance. A specific example of the implementation of the theory is disclosed in Japanese Patent No. 2605926. In the example, the face portion of the golf club is 2 mm to 3.5 mm thick, the crown portion is 0.6 mm to 2 mm thick, and the sole portion is 1 mm to 3 mm thick. The face portion is broadened (40 mm or longer in vertical length and 70 mm or longer in horizontal length). The golf club thus dimensioned succeeds in stabilizing the ball flying direction and increasing the ball flying distance.
Thus, the thinning and enlarging of the face portion and the like improve the characteristics of the golf club indeed, but suffers from the following problems. Particularly the face portion (i.e. the ball striking surface) is liable to broken. A crack, formed in the ball striking surface, grows through its long time use. In other words, its durability is not satisfactory. For this reason, there is a limit in increasing the size of the club head, and hence reducing its weight and adjusting a weight distribution in the club head.
Proper selection of the material for the club head and the method of manufacturing the same may solve those problems to some extent. For example, where a .beta. alloy is used for titanium, the face portion, for example, may be reduced in thickness since its strength is higher than that of a pure Ti .alpha. alloy and .alpha..beta. alloy. When the rolling is used for manufacturing the club head, the crystal grains are fined and increased in density, to increase a strength of the club head.
Use of those techniques still fails to achieve a satisfactory thinning and enlarging of the face portion. The reason for this will be described with reference to the related drawings attached to this specification.
Reference is made to FIGS. 1, 2(a) and 2(b) for explaining the whole golf club. Of those figures, FIG. 1 is a view showing the whole construction of a golf club. FIG. 2(a) is a front view showing a club head of the golf club, and FIG. 2(b) is a cross sectional view taken on line A--A in FIG. 2(a).
In those figures, reference numeral 1 is a shaft; 2 is a grip; and 5 is a club head. The club head 5 has a neck 6 to which the shaft 1 is attached, and its configuration is defined by a face portion (a ball striking surface) 7, a top-side (crown) portion 8, a toe-side portion 9, a heel-side portion 10, a sole portion 11, and a back-face portion 12. Those surface portions are demarcated by ridge lines 15. Score line grooves 7a are formed in the face portion 7 to impart a spinning motion to a ball when striking the ball.
A deformation state of the club head when the face portion 7 of the club head strikes a ball will be described with reference to FIG. 3. As shown in FIG. 3, as the result of the recent tendency of size increasing, the face portion 7 of the club head is configured to be short in height and long in width, and its depth (thickness) is h. In the area serving as a sweet spot of the face portion 7, the horizontal length X is longer than the vertical length Y.
At the instant that the face portion 7 thus configured impacts against the ball, the face portion 7 is entirely deformed (deflected) toward the back-face portion. Specifically, an impact produced when striking the ball deforms the face portion 7 in the X and Y directions. In this case, an amount of deformation X1 in the X direction is not equal to that Y1 in the Y direction. The deformation amount Y1 is larger than the deformation amount X1 since the dimension of the face portion when viewed in the X direction is larger than that in the Y direction at the center P of the sweet spot when the deformation amounts are measured per unit length.
Thus, the deformation amount per unit length in the vertical direction is larger than that in the horizontal direction. Because of this, the face portion is liable to crack in the horizontal direction. With the increase of the head size, the horizontal size of the club head is larger, so that the deformation amount per unit length increases to further promote its fissuring.
The present invention was made in view of the facts that, when an impact is applied to the face portion, the deformation amount per unit length at the center on the face portion differs with the directions, viz., the deformation amount in the long-dimension direction of the face portion is different from that in the short-dimension direction, and that this hinders the thinning of the club head.
With the increase of the club head size, the face portion, for example, of the golf club is configured such that the horizontal length (the length in the long-dimension direction) is increased. Therefore, crack and breakage in the horizontal direction is liable to be formed in the face portion. In designing the conventional golf club, the above discovery is not taken into consideration, and a conventional measure taken for the crack formation problem is to merely increase the thickness of the face portion. The conventional technique is confronted with difficulties of reducing in thickness those surface portions and other surface portions of the club head for the reason that the thinning of those surfaces leads to formation of crack.
Accordingly, an object of the present invention is to provide a golf club with a club head which may be reduced in its thickness with no fear of impairing the durability and strength of the club head.
(2) As disclosed in Japanese Patent Laid-Open Publication No. Hei-6-269518), if the ball is greatly elastically deformed when it is hit, the energy imparted to the ball is consumed for the motion to restore the deformed ball to its original form. As a result, the flying distance of the ball is not increased.
To prevent the face portion of the club head from being deformed inward and permanently deformed so when hitting the ball, a ratio of a durability of .sigma. of the face portion to an elastic modulus (Young's modulus) E thereof (.sigma./E) is set at 5.times.10.sup.-3 or larger. In other words, when hitting the ball, the face portion is made elastically deformed inward, whereby an elastic deformation of the ball is minimized to thereby increase the flying distance of the ball.
Only increasing of a strength of the face portion to withstand some amount of deformation of the face portion fails to optimize a coefficient of rebound or restitution of the face portion when hitting the ball, to increase the flying distance, and to secure a directional stability of the ball. To prevent an extreme deformation of the ball when the face portion impacts on the ball and to optimize the coefficient of restitution of the face portion, it is necessary to adjust a flexure amount of the face portion. If a flexure amount of the face portion when hitting the ball is calculated in advance and the face portion is designed to have an optimum flexure amount when hitting the ball, it is possible to optimize the coefficient of restitution of the face portion, to increase the flying distance of the ball, and to secure the directional stability of the ball.
Through the investigation on the flexure characteristic of the face portion of the club head, the facts were discovered in that a flexure amount of the face portion per unit length when hitting the ball is larger in the vertical direction (the short-dimension direction) of the face portion than in the horizontal direction (the long-dimension direction), and that a flexure amount of the face portion depends greatly on the conditions of the face portion in the vertical direction. The present invention was made in view of these facts, and an object of the invention is to provide a club head of a golf club which is configured so as to have an optimum flexure amount of the face portion of the club head in the vertical direction, whereby a coefficient of restitution of the face portion when hitting the ball is optimized, the flying distance is increased, and the directional stabilization of the ball is secured.
(33) Generally, the club head of the gold club can be thinned using a material of high strength, and be increased in size and reduced in weight. The theory teaches that the size increase of the club head increases its inertia moment and enlarges its sweet spot, and weight reduction of the club head leads to increase of its swing speed, and as a result, the directional stability of the ball is secured and the flying distance of the ball is increased. For this reason, recently, various kinds of materials of high strength are sued for the club heads. With use of those kinds of materials, a designer can design club heads with an increased design freedom while satisfying various characteristic requirements.
The face portion of the club head is flexed by an impact produced when striking a golf ball. Therefore, it is liable to flaw and to be worn, and will crack through a long time use, and is inferior in durability to other portions of the club head. For this reason, for the face portion of the club head, such a material, e.g., stainless or titanium, which is different from that of the club head, as to withstand an impact produced when striking the ball, is processed by forging or the like to form a face plate (the same material as of the club head may be used if it is able to withstand the impact). The face plate is mounted on the club head. Reduction of the face plate in weight accrues to increase of the gravity center depth, and hence increase of the sweet spot, as in the case of the weight reduction of the head body. Accordingly, a design freedom in designing the club head is increased.
As is known, a fiber reinforced metal (FRM) as a metal reinforced with reinforced fibers in order to increase a strength of a material constituting the face plate and to reduce the weight thereof, is used for the material of the face plate. Japanese Patent Laid-Open Publication No. Hei-8-280855 discloses a composite reinforced material in which titanium, aluminum alloy or the like is used for a matrix, and reinforced fibers made of silicon carbide, boron or the like is used for a reinforcing material.
Since the FRM is such that a metal is reinforced with reinforced fibers, it is desirable that a percentage of the reinforced fibers contained in the matrix is large. In a case where to form an FRM, a matrix is a material suitable for the face plate, e.g., aluminum, titanium, stainless or the like, and reinforced fibers is mixed into the matrix, a process of high temperature and high pressure is inevitably carried out. Therefore, the process possibly gives rise to change of properties of the reinforced fiber, oxidization of the material and the like. The result is to loosen the bounding of the matrix to the reinforced fibers, to generate air bubbles in spaces between the matrix and the reinforced fibers, and to reduce a strength of the material. As a consequence, an attempt to increase the percentage of the reinforced fibers contained in the matrix is rejected.
To bring out the best in the reinforced fibers of the composite reinforced material, a preferable material for the matrix is relatively soft and easy to interdiffuse, good in wetting properties with the reinforced fibers, and lower in melting point than the reinforced fibers.
As already stated, the face plate is flexed by an impact produced when hitting the ball, and the resultant flexure causes a bending stress in the face plate. A magnitude of the bending stress is proportional to a distance from the neutral axis of the face plate, viz., it increases as the distance increases. The face plate made of FRM may be increased in its strength, thinned in thickness, and reduced in weight in a manner that a reinforced fiber layer is disposed apart from the neutral axis as much as possible to increase a rigidity of the fiber layer contained portion.
Where such a material as to be relatively soft and easy to interdiffuse, good in wetting properties with the reinforced fibers, and lower in melting point than the reinforced fibers, is used for the matrix, the face plate made of the material is easy to be worn by an impact produced when hitting the ball, and hence is unsatisfactory in durability. Locating the reinforced fiber layer close to the hitting surface thereof creates some problems. A portion close to the hitting surface is worn by the impact, so that the reinforced fiber is liable to be exposed there. The exposed reinforced fibers impair the look of the club head, and possibly cause-crack in the club head. The crack of the club head reduces a strength of the club head. For this reason, there is a limit in reducing a distance of the reinforced fiber layer to the hitting surface.
Accordingly, an object of the present invention is to provide a structure of an FRM face plate mounted on the club head of a golf club, which the structure allows the reinforced fiber layer to be located close to the hitting face of the club head.